Air-speed computer



Feb. 29, 1944. E. KoTcHER AIR SPEED COMPUTER Filed Feb. 25, 1941 ence inpressure take into consideration Patented Feb. 29, 1944 A uNiTl-:nSTATES PATENT ortica Ara-SPEED' COMPUTER am Komm, Dayton, ohioApplication February 25, 1941, Serial No. 380,485 l (ci. zassn 1 Claim.

( Granted The invention described herein may be manufactured and used byor for the Government for governmental purposes, without the payment tome of any royalty thereon.

This invention relates to `computers and more particularly -to acomputer for use in quickly and accurately determining the true airspeed of a moving body-such as an airplane-from data indicated oninstruments carried by the airplane. In view of the increasingimportance of accurate nicht research, aerial navigation and bombing, itis in some instances imperative that the true air speed of aircraft beknown to a high degree oi accuracy.'

Because of many existent sources of error, a great deal of diiculty hasbeenexperienced in accurately determining the true air speed of anairplane and especially at speeds in excess oi 250 miles per noun,Certain of these errors, such as mechanical and installation errors maybe evaluated to a large extent by timed flight over a measured speedcourse. Errors due to inherent characteristics of the instrumentsresulting from the difference between the characteristics of thestandard sea level air assumed for marking the diai and the Iair that isactually encountered in flight, may be corrected by theoretical methodsoi computation which are the subject matter of the present invention;

In the present form oi air-speed indicators, the indicator needleoperates as the result of a. differbetween the Pitot tube and the statictube, this diiierence in pressure being equal to the impact pressure..Until recently the compressibility of air was ignored in the calibrationof air-speed indicators as it also was in air-speed computers used inconverting to true air speed the air speed indicated at conditions otherthan standard sea level atmospheric conditions. However, it has beenfound that with the advent of modern high speed airplanes it isnecessary-4in order to accurately calculate true air speed-to theadiabatic compressibility oi air coming to rest. In that case the im-4pact pressure AP'for operating the air-speed indicator may be given bythe formula c=ve1ocity of sound=45vm P. 'H'

under the act of amended April 30, 1928;

T being absolute temperature in centigrade .equating impact pressures atsea .March 3, 1883, as 370 0. G.' '157) Machs Number Vis well known inthe sciences dealing with the compressibility of air such as in thestudy of shock waves caused by Ybullets travelling at supersonic speedsand the study oi airfoil characteristics at very high speeds approachingthe velocity of sound.

Instruments for indicating' air speed are now calibrated under standardconditions in accordance with the same theory used to develop the abovestated formula and it is the purpose of the present invention to providea computer for converting indicated air speed observed under atmosphericconditions other than standard to true ai; speed. By way of explanation,standard pressure altitude represents a fictitious altitude for a givenactual pressure observed at an actual altitude according to acceptedrelationships between pressure and altitude. The standard atmosphere hasbeen established in order to reduce airplane performance under dicrentatmospheric conditions to a common basis o! comparison with respect to astandard average variation of atmospheric conditions of pressuredensity,and temperature with altitude. l

The theory in back oi the construction of the computer is as follows:

Since true 'velocity is a function of impact pressure a given impactpressure and its corresponding indicated air speed for sea ieveistandard atmospheric conditions represents different true velocities fordiiIerent atmospheric conditions.

True velocity at conditions other than the stand.

ard conditions u nder which the air-speed indicators are calibrated maytherefore be computed by y ievel to altitude condition as follows:

Pict-raider k s Pu #eert-1i where V v P=the standard sea levelatmospheric pressure :2116.4 lbs. per sq. it. P=the actual atmosphericpressure at, the altitude where the speed is being measured c=thevelocity of atmosphere=783 M. P. H. c=the velocity .of sound in M. P. H.at the alti- -tude where the night is being measured,

and is equal to degrees where Tis in centisrade degrees k=theadlabatic'exponent=1.40 for air inM. P. H. P. H. which is t0 be Vi=theindicated airspeed Vt=the air speed in M.

sound in the standard sea level In the above equation the quantity V,'Vi E Tos which is also equal to the indicated Machs Numbei* Ml and L.0222 V, c *1t/T +273 which is equal to the true Machs Number Mt. Theabove equal values may be substituted in the above equation along withknown standard sea level quantities and the equation becomes a factorfor temperature observed at the given altitude.

It will be noted that the above equation com prises three factors, i.e.,

nl@ factor within the left hand bracket; and the factor within the righthand bracket. The rst factor may be evaluated for any standard pressurealtitude 4by a simple division; factor 'may be evaluated for any knownindicated air speed; and the product factors will result in anevaluation of the third factor Within the right hand bracket. In view ofthe fact that the equations within the left and Vright hand brackets areidentied in form, it is computer by which an as Me Within one of thelpossible to formulate a unknown variable such brackets may be solved fordirectly in terms of the other Avariable quantities in the equation.vThis result is accomplished by the hereinafter described physicalembodiment of the computer in which,

. The single iigure in the drawing is a plan View or' the computerconstructed in accordance with the invention.

bers i0, M and 241 are rotatably mounted for relative movement on apin-like member 22. Meni- Iber i has located thereon an altitude scale Aand a. temperature scale T. Member i0 is provided with an indicated airspeed scale Vi, a Machs Number scale scale Vt.

A and T scales are provided with a common indexing line 2d.

The construction, and cooperation of coordination, correlation thescales will new be described.

The A scale constituting the first factor or 2116.4 P

is constructed by dividing 21-16.4 by the pressure P correspondingl tothe standard altitude as indicated by an altimeter, locating this lattervalue on the line markings of a standard logarithmic the secondof therst and second suit 1.453 is obtained. On the zero altitude or 1.00 onthe standard logarithmic i the altitude scale as shown in the drawingis, in the preferred embodiment of the invention, from 1000 ft. to36,000 ft.

or a similar logarithmic scale f2@ 0.502.

range oi speeds ex'- In view of the identity of the form of theequations fact that coordinated with the Vx value is a corremondingscale of M values, it follows that Ms may be determined directly by amultiplication operation between the A and Vi scales.

The equation may now be solved directly for Mi as in the followingexample for an indicated air speed V1 of 350 M. P. H. (corresponding toaltitude of 20,000 ft. In order to true Machs Number Mt, set the indexarrow of the A scale opposite Vi scale, then set an indicator over onthe A scale and read under the indiori the M scale the value 661, i. e.,

20,000 cator line Mt=0.361. Knowing Mt it is now desired to calculateVi. Since Vt is equal to V Mn/T+273 to construct on the computer, scalesfor solving the above equait is now necessary the Vt and T tion.

The Vt scale is a standard logarithmic scale and is constructed asfollows: In vthe process of determining the true air speed Ve, it isnecessary to transfer to the Vt scale the Mr value as found on the Mscale. Therefore, in order to facilitate this transfer the logarithmicVt scale is marked res si to those chosen for the M the Vr scale may beoriented permit the torliy on the M scale for 340 proper Achoice ofrange on the Vi scale. With a and modulus, the M, and

the values represented on the M and Vr de or to be so nearly incoincidence as to The temperature index arrow is chosen so as tocorrespon scale which must be of the same modulus or unit l employed forthe Vc scale since a multiplication processl must be perdii'ferentvalues of T in centigrade degrees, the value of V T+2'13 is determinedand also located on this logarithmic scale, but identified by thecorresponding temperature T. For example, for the temperature of C., thevalue of \/,20+2l3 is equal to 15.9. Within the same unit length oflogarithmic scale was used to locate the temperature index arrow. thevalue of 15.9 is located but identified C. For simplicity ofconstruction. the T scale is same-moving element as the A same indexarrow is used for both the temperature and altitude scales.

be combined into a single scale by d to standard logarithmic theinvention True air speed may be obtained by means of the Vt and T scalesas follows'. For instance, assume the previous example of an indicatedair speed of 350 M. l. H. observed at 20,000 ft. where, in addition, theobserved air temperature was found to be C. The value on the M scalecorrespondin true Machs Number indicator temperature scale is moveduntil the temperature index arrow is also under the indicator line. Theindicator line is then moved to coincide with the temperature of -30' onthe T scale and the ir peed, Vt is found under the indicator varyingaltitudes, and having of said altitudes, a secon scale cooperating withthe rirst scale and positioned on the other of vsaid movable members.said second scale expressing multiplying factor which corrects thestandard give the value of the a third scale immovably positioned withrespect to saidsecthird scale expressing the ratio of ond scale, saidthe true air speed to the velocity of sound at the correspondingaltitude, a fourth logarithmic scale positioned on said other membercarrying logarithnrically arranged indicia, said indicia reprethetransferred values of the third and a fifth logaon said one movable withsaid fourth scale,

expressing the velocity of sound identined thereon terms of the airtemperature, which scale, in cooperation w th the transferred value ofthe third scale to the fourth scale, gives the true air speed on thefourth scale.

EZRA KQTCHER.

